1. Field of the Invention
This invention relates to an apparatus for detecting a condition of burning in an internal combustion engine.
2. Description of the Related Art
It is known to control a spark timing in an internal combustion engine in response to a knock condition thereof. Generally, such knock control uses a sensor for detecting engine vibration as an indication of an engine knock condition. However, engine vibration is caused by not only knock but also other factors. The vibration-based knock sensor tends to output an inaccurate signal when engine vibration is caused by a factor other than knock.
Japanese published unexamined patent application 58-7536 discloses that the ion current, which is generated between the electrodes of an engine ignition plug due to detonation waves, is detected by an ion current sensor. The output signal of the ion current sensor has an amplitude and a width which vary with the intensity of generated engine knocking. Therefore, the occurrence of the knocking and the intensity of the knocking are detected directly and accurately without recognizing the vibration of the engine as the knocking.
U.S. Pat. No. 4,672,324 discloses a flame protection circuit having a first input terminal for a flame probe and a second input terminal for a burner bed. The circuit in U.S. Pat. No. 4,672,324 includes an alternating voltage source and a parallel-combination of a resistor and a capacitor connected to the input terminals. In the presence of a flame between the burner bed and the flame probe, an ionization current flow has a direct current component because of the rectifying effect of the flame. The direct current component produces a measuring direct voltage across the capacitor. The circuit further includes a comparator which compares the measuring direct voltage with a reference voltage to produce a final output signal. The final output signal represents whether or not the flame is present.